The invention relates generally to liquid crystal display panels and, more particularly, to a method of making low-resistivity source and gate lines on LCD glass panel substrates using pure aluminum metal.
The length and density of bus-line conductors, such as source and gate lines, on large-size high-resolution liquid crystal display (LCD) panels present special manufacturing problems. As LCD panels grow in size, the bus-line conductors become longer. Long conductors formed on glass, such as the gate lines in bottom-gate thin film transistor (TFT) LCD matrices, exhibit problems associated with the resistivity of the conductive material, including signal delays and pulse distortion. Metals such as aluminum appear suitable for use as bus-line conductors on 18+ inch active-matrix liquid crystal display (AMLCD) panels having XGA resolution or higher. Pure aluminum offers the advantage of low resistivity (approximately 3.0-.mu..OMEGA.-cm) but its use as a TFT bus-line material on glass presents practical problems.
One problem associated with the use of pure aluminum for LCD gate and source lines is the formation of undesirable surface features, known as hillocks, when the deposited aluminum is heated above 200.degree. C. during subsequent processing. Hillocks are caused by a mismatch in the thermal expansion coefficient between the glass and the metal conductor. Stresses in the aluminum are released as hillocks when the substrate and metal are heated. Hillocks are a major impediment to the use of aluminum lines on glass because, for certain TFT architectures, the aluminum conductors must be covered with a layer of dielectric material, for example, the gate dielectric layer. Deposition of the dielectric layer involves processes which require the substrate to be heated above the aluminum yielding point (150.degree. C.-200.degree. C.). That is when hillocks are formed. Manufacturing yields suffer because hillocks can form shorts across the conductors, destroying devices.
One prior art approach to resolving the hillock problem is to form bus lines out of aluminum alloys instead of pure aluminum. Alloys of aluminum and tantalum (Ta), neodymium (Nd), zirconium (Zr), copper (Cu) and tungsten/molybdenum (W/Mo) all have less susceptibility to hillock formation at conventional process temperatures than does pure Al. However, such alloys do not have the low resistivity of pure aluminum. They also present handling problems during deposition and performance problems due to non-uniform distribution of the constituent metals.
Yet another approach to limiting hillock formation on pure Al bus lines is to cap the pure aluminum with a layer of a suitable metal such as titanium (Ti). Capping greatly reduces hillock formation and also is effective against corrosion. The principle disadvantage of capping is the additional process steps involved, which increases costs and lowers yield.
It would be preferable to find a method of forming pure Al conductors on glass without using a capping layer. Aluminum can be made more resistant to subsequent hillock formation if the Al is formed on a non-aluminum underlayer of titanium (Ti). Titanium film which exhibits a strong (001) texture tends to impart a strong (111) crystalline texture to an overlying aluminum layer. Studies have indicated that strongly textured (111) aluminum films demonstrate higher resistance to hillocking than weakly textured or randomly oriented films.
Still another factor which influences the susceptibility of aluminum film to hillock formation is the surface texture of the glass substrate on which the gate lines and other conductors are deposited. A typical transparent insulating substrate for use in LCD panels is 1737 Corning Code glass (untreated). Such glass, referred to herein as untreated glass, generally has a surface roughness (the height of measured surface features) of approximately 2.5 .ANG.. An alternative glass substrate product available for LCD panels is 1737 glass that has been subjected by the manufacture to a dilute acid solution and, hence, is referred to as "treated" glass. The surface roughness of treated glass is approximately 5.1 .ANG.. It has been found that the use of treated glass as a substrate for the deposition of Ti/Al layers reduces hillock formation during subsequent heating. It is believed that the greater surface wetness of the treated glass promotes the formation of voids at the interface between the aluminum layer and the titanium underlayer. The voids possibly allow for some stress relief in the plastic deformation of the aluminum during subsequent heat treatment, effectively providing an additional pathway for stress relief other than hillock formation. Whatever the actual mechanism, acid treated glass does appear to reduce hillock formation on the aluminum surface of Al/Ti films. Nevertheless, the extra cost of treated glass in large-scale manufacturing of TFT panels favors the use of untreated glass, if other methods of hillock control can be found.
It would be most advantageous to be able to provide low-resistivity pure aluminum conductive lines on LCD glass panels, with acceptably low hillock densities after deposition of a surface dielectric layer, using the lower-cost untreated glass substrate.
It would also be advantageous to form conductive lines containing pure aluminum on LCD glass panels in which hillock densities are acceptably low, after subsequent heating to form a surface dielectric layer, without the additional cost and complexity of forming a capping layer of titanium or another suitable metal on the Al.
It would be additionally advantageous to provide gate lines and other connectors on glass substrates formed of highly oriented (111) aluminum, which is resistant to hillock formation, using a process which provides for the deposit of silicon nitride or another dielectric material on the conductors but which does not require a process temperature above 300.degree. C.
Accordingly, the present invention provides a method of forming conductors covered with a dielectric layer on glass or another transparent insulating substrate for use in forming active devices in liquid crystal displays. The method comprises a step of forming a conductive sheet having at least two layers on the substrate. The conductive sheet includes an underlayer of titanium, closest to the substrate, having a thickness generally in the range of 50 .ANG. to 500 .ANG.. The titanium underlayer is overlaid by a layer of aluminum having a thickness generally in the range of 750 .ANG. to 2000 .ANG.. The method further includes a step of forming a mask pattern on the conductive sheet to mask areas in the form of lines and conductors. The masked areas protect the conductive sheet material from removal during subsequent etching steps. The method further includes the step of etching selected areas of the conductive sheet material using an ion etch chamber. The etching proceeds in the areas unprotected by the mask pattern formed in the proceeding steps. The etching step uses an etch process which etches both titanium and aluminum so that both are removed in a single etch. Consequently, the conductive sheet material which remains following the etching step includes conductors in the form of lines on the substrate. The method additionally includes the step of depositing dielectric material on the conductive lines formed in the etching step. Dielectric material is deposited using a vapor deposition process which has a maximum process temperature below 370.degree. C., and preferably at or below 300.degree. C. As a result of the vapor deposition process, the conductors and lines on the substrate are covered with a dielectric layer to provide low-resistivity signal lines and connectors, including source and gate lines, extending between and incorporated into active devices and liquid crystal displays.
The invention is particularly directed to a method of forming conductive lines having an underlayer of titanium and an adjacent top layer of aluminum on the surface of standard (untreated) glass suitable for use in LCD displays. In its preferred embodiment, the method includes depositing a sheet of titanium between 300 .ANG. and 500 .ANG. thick on the glass, overlaid by an aluminum layer deposited on the titanium having a thickness between 1,000 .ANG. and 1,200 .ANG.. The specified thickness of titanium yields strongly textured (001) or (002) titanium film on the glass which, in turn, is known to promote the formation of strongly textured (111) aluminum in the specified range of thicknesses. The (111) aluminum is known to be more resistant to hillock formation during subsequent heating when a dielectric layer is deposited in accordance with the present invention.
The step of etching the aluminum Al/Ti sheet material in accordance with the present invention is preferably carried out in a reactive ion etch (RIE) chamber and comprises the following steps: i) a breakthrough step in which BCl.sub.3 is introduced into the chamber; ii) a main etch to remove aluminum and titanium from the unmasked areas by introducing Cl.sub.2 and BCl.sub.3 into the chamber; and iii) introducing CF.sub.4 into the chamber to remove residual etching byproducts.
The preferred method of depositing dielectric material on the substrate and the resultant conductive lines formed in the etching step include vapor deposition of silicon nitride (SiN.sub.x). Preferred vapor deposition methodologies include Plasma Enhanced Chemical Vapor Deposition (PECVD) or Physical Vapor Deposition (PVD, or "sputtering"). The method preferably employs preheat and silicon nitride deposition process temperatures which do not exceed 300.degree. C. As a result, the methodology of the present invention provides conductors formed of pure aluminum metal on a glass substrate, and coated with a dielectric layer of silicon nitride, in which hillock densities are minimized to within acceptable manufacturing tolerances.